Differential pressure responsive devices responding to the pressure drop across filters to provide an indication of a predetermined collection of foreign material on the filter are old such as shown in the U.S. Murrell F. Kautz et al U.S. Pat. No. 3,027,865 issued Apr. 3, 1962. In systems using differential pressure responsive devices such as that disclosed in the Kautz patent whether in a forced air heating and/or cooling system or in a range hood vent system, the flow of air has been relatively constant.
The present invention is concerned with the compensation of a differential pressure responsive filter condition indicator for changes in the rate of flow of the medium flowing through the filter. Specifically, a differential pressure responsive device responding to a condition indicative of flow is connected to compensate for the effects of changes in flow on the differential pressure responsive device responding to the pressure drop across the filter. As the flow of medium through the filter changes a flow signal will be obtained and an output signal will be truly indicative of the condition of the filter.